The present invention relates to a laser diode light emission sensing circuit.
FIG. 1 shows a constitution of a conventional laser diode light emission sensing circuit. The conventional laser diode light emission sensing circuit shown in FIG. 1 comprises a laser diode 20, a photodiode 22 for sensing a luminous state of the laser diode 20, a current/voltage converting circuit (hereinafter referred to as xe2x80x9cI/V converting circuit) 23, a reference voltage source 24 for producing a reference voltage, a comparator 25 for comparing a voltage signal from the I/V converting circuit 23 with the reference voltage from the reference voltage source 24, a time constant circuit 26, and an output terminal 32. Furthermore, the time constant circuit 26 comprises a transistor 27, a resistance 28, a capacitor 29, and a comparator 31, and a reference voltage source 30.
The laser diode light emission sensing circuit having the above described constitution functions to sense a luminous state of the laser diode 20 such that a lightwave signal of the laser diode 20 is received by the photodiode 22, a current signal output of the photodiode 22, is converted into a voltage signal by means of the I/V converting circuit 23, the voltage signal converted is compared with a reference voltage from the reference voltage source 24 in the comparator 25, and further a logical output of the output terminal 32 is maintained by the use of the time constant circuit 26. The time constant circuit 26 is required to maintain an output after detecting the output of the laser diode 20 even if logic 0 continues for a certain period of time in the laser diode 20.
However, since the above-mentioned laser diode light emission sensing circuit maintains a logical output of the output terminal by employing the time constant circuit, there is a problem of realizing downsizing of apparatus.
Since time constant circuit maintains an output in an analog fashion, it is necessary for a capacitor having a high capacity. Such capacitor requires a capacitive value of 100 pF or high in order that logic 0 maintains at 72 bits or more in a signal having a bit rate of 150 Mbps, so that a size of chip becomes bulky.
Moreover, a length of burst signal produced in synchronous with a light emitting timing or a light outing timing in a laser diode is generally about 2.5 xcexcsec., a guard period of time between the burst signals is 26 nsec., and a discharge time constant of the above-mentioned time constant circuit is about 1.5 xcexcsec. As shown in FIG. 2, when a laser diode does not emit light because of a trouble which happens in process of burst signals, such trouble cannot be detected up to a point in the middle of the burst signals from discharge time constant.
The present invention has been made in view of the above-mentioned problems involved in the prior art. Accordingly, an object of the present invention is to provide a laser diode light emission sensing circuit which can reduce a size of the resulting apparatus, and can shorten a period of time required for detecting a trouble in the laser diode.
According to the first feature of the invention, a laser diode light emission sensing circuit for sensing a luminous state of the laser diode, comprises:
a burst signal producing means for producing a burst signal which changes to a first state in synchronous with light emitting timing of the laser diode, while which changes to a second state in synchronous with light outing timing of the laser diode; and
a maintaining means which maintains a prescribed output state representing a luminous state of the laser diode in synchronous with changing to a first state of the burst signal in a term of a third state of a CELL signal which changes to the third state before starting a light emission term wherein the laser diode repeats continually light emission and light outing, and which changes from termination of the light emission term to a fourth state, the maintaining means cancelling the prescribed output state maintained in synchronous with changing to the fourth state of the CELL signal.
In the above described laser diode light emission sensing circuit, the maintaining means may be a delay flip-flop which utilizes the burst signal from the burst signal producing means as a clock input, while which utilizes the CELL signal as a reset input, and maintains a data input always at a high level.
In the above described laser diode light emission sensing circuit, the maintaining means may contain an NOR circuit which utilizes the burst signal and the CELL signal as inputs, and a set-reset flip-flop which utilizes the output from the NOR circuit as a set input, while which utilizes the CELL signal as a reset input.
According to the second feature of the invention, a laser diode light emission sensing circuit for sensing a luminous state of the laser diode, comprises:
a light sensing means for sensing a lightwave signal of the laser diode to convert it into a current signal,
a current/voltage converting means for converting a current signal from the light sensing means into a voltage signal,
a comparing means for comparing a voltage signal from the current/voltage converting means with a reference voltage from a reference voltage producing means to output a predetermined signal when the voltage signal from the current/voltage converting means exceeds the reference voltage from the reference volt age producing means,
the reference voltage producing means for outputting the reference voltage to the comparing means,
a first output inverting means for inverting an output of a CELL signal rising before starting a light emission term wherein the laser diode repeats continually light emission and light outing, and falling from termination of the light emission term, and
a delay flip-flop which utilizes an output of the comparing means as a clock input, while which utilizes an output of the first output inverting means as a reset input, and maintains .a data input always at a high level.
According to the third feature of the invention, a laser diode light emission sensing circuit for sensing a luminous state of the laser diode, comprises:
a light sensing means for sensing a lightwave signal of the laser diode to convert it into a current signal,
a current/voltage converting means for converting a current signal from the light sensing means into a voltage signal,
a comparing means for comparing a voltage signal from the current/voltage converting means with a reference voltage from a reference voltage producing means to output a predetermined signal when the voltage signal from the current/voltage converting means exceeds the reference voltage from the reference voltage producing means,
the reference voltage producing means for outputting the reference voltage to the comparing means, and
a delay flip-flop which utilizes a CELL signal falling before starting a light emission term wherein the laser diode repeats continually light emission and light outing, and rising from termination of the light emission term, as a reset input, while which utilizes an output of the comparing means as a clock input, and maintains a data input always at a high levels.
According to the fourth feature of the invention, a laser diode light emission sensing circuit for sensing a luminous state of the laser diode, comprises:
a light sensing means for sensing a lightwave signal of the laser diode to convert it into a current signal,;
a current/voltage converting means for converting a current signal from the light sensing means into a voltage signal,
a comparing means for comparing a voltage signal from the current/voltage converting means with a reference voltage from a reference voltage producing means to output a predetermined signal when the voltage signal from the current/voltage converting means exceeds the reference voltage from the reference voltage producing means,
the reference voltage producing means for outputting a reference voltage to the comparing means,
a second output inverting means for inverting an output of a CELL signal rising before starting a light emission term wherein the laser diode repeats continually light emission and light outing, and falling from termination of the light emission term,
a third output inverting means for inverting an output of the predetermined signal from the comparing means,
an NOR circuit utilizing a signal from the second output inverting means and a signal from the third output inverting means as inputs to output a predetermined signal when both the signals are at a low level, and
a set-reset flip-flop utilizing an output of the NOR circuit as a set input and utilizing a signal from the second output inverting means as a reset input.
According to the fifth feature of the invention, a laser diode light emission sensing circuit for sensing a luminous state of the laser diode, comprises:
a light sensing means for sensing a lightwave signal of the laser diode to convert it into a current signal,
a current/voltage converting means for converting a current signal from the light sensing means into a voltage signal,
a comparing means for comparing a voltage signal from the current/voltage converting means with a reference voltage from a reference voltage producing means to output a predetermined signal when the voltage signal from the current/voltage converting means exceeds the reference voltage from the reference voltage producing means,
the reference voltage producing means for outputting the reference voltage to the comparing means,
a third output inverting means for inverting an output of the predetermined signal from the comparing means,
an NOR circuit utilizing a CELL signal falling before starting a light emission term wherein the laser diode repeats continually light emission and light outing and rising from termination of the light emission term as well as a signal from thexe2x80x9d third output inverting means as inputs to output a predetermined signal when both the signals are at a low level, and
a set-reset flip-flop utilizing an output of the NOR circuit as a set input and utilizing the CELL signal as a reset input.